1. Field of the Invention
The present invention relates to a heater for heating a fluid intended to pass through a fuel tank canister.
2. Description of the Related Art
Fuel vapours escaping from a fuel system are a source of hydrocarbon emissions in motor vehicles. Fuel leaks due to lack of sealing and also losses by evaporation are most particularly the subject of increasingly stringent regulations, especially for applications in motor vehicles.
Now, fuel tanks, in particular for motor vehicles, usually have a venting orifice for balancing the internal pressure with the atmospheric pressure, for example during variations in the fuel level resulting from filling the tank or from the consumption of fuel by the engine, or else during variations in temperature. Therefore, to meet the aforementioned standards, this venting orifice is conventionally connected to atmosphere via a duct and a chamber, usually called a canister, which contains a substance that adsorbs the fuel vapours, this usually being activated charcoal. The role of the canister is to prevent the release of fuel vapours into the atmosphere. The canister is regularly purged, i.e. the vapours are desorbed, generally by means of a stream of air, and sent to the air intake system of the engine in order for them to be burnt therein.
A major problem with systems for controlling evaporative loss using activated charcoal as adsorbent material in the canister is that, under low ambient temperature conditions, it is difficult for the fuel that has been adsorbed to be released and purged from the canister. It has been determined that, with the adsorbents currently used, satisfactory release of the stored fuel occurs at the ambient temperature or above it. When the temperatures drop well below the ambient temperature, the effectiveness of the system is significantly impaired. It is therefore advantageous to increase the temperature inside the canister. The improvement in desorption has the consequence that the adsorbent is more active, as it has been better desorbed.
Heaters located outside the canister for heating the adsorbent, are known from the prior art. Document U.S. Pat. No. 6,230,693 discloses an auxiliary canister designed to be connected to the vent of a main canister, said auxiliary canister containing, like the main canister, an adsorbent material. This auxiliary canister includes a heater consisting, for example, either of a PTC (Positive Temperature Coefficient) ceramic extruded in the form of a honeycomb, or of aluminium fins attached to a bar made of PTC-type ceramic. Such a canister has the drawback of heating the adsorbent and not the air flowing when the main canister is being purged and, in addition, the drawback of introducing additional pressure drops in the flow of purge air.
Also known, as described in Patent CA 1271797, is a heater that is fixed to a canister and in which the air passes around heated plates. The heater is fixed to the canister so as to close off one end of the latter. The geometry of the heater depends closely on that of the canister and cannot be standardized so as to be fitted to canisters of various geometries.